1. Field of the Invention
The present invention relates to a voltage drop DC-DC converter and, in particular, to a voltage drop DC-DC converter for use as a power source circuit of a portable telephone set of such as PHS (Personal Handyphone System) or other portable telephone system or a portable electronic device such as electronic book or PDA (Personal Digital Assistance), which DC-DC converter is high efficiency with low noise and occupies a small area.
2. Description of the Prior Art
In a conventional battery-powered electronic device such as portable telephone set of a portable telephone system or a portable electronic device, an operating voltage thereof is lowered in order to reduce power consumption thereof and, recently, such battery-powered electronic device is operated with a 1.8V power source since a LSI mounted thereon is operated with a 3V power source. On the other hand, the development of LSI operating with a 1.5 V power source has been started most recently.
Incidentally, an output voltage of a power source such as a lithium ion battery to be used in an electronic device of such kind is in a range from 3.0V to 4.2V and is usually about 3.6 V.
The operating voltage of such LSI is usually obtained by dropping a battery voltage. In the conventional series regulator including a transistor for dropping a battery voltage, voltage-dropping efficiency is as low as about 50%.
On the other hand, the voltage-dropping efficiency of a voltage-dropping circuit constructed with a charge-pump circuit and a voltage-dropping transistor is 50% or higher. Therefore, such voltage-dropping circuit has been used as a power source circuit of a battery-powered electronic device.
A conventional voltage drop DC-DC converter of switched capacitor type, which is an example of such voltage-dropping circuit, is disclosed in JP H8-205524A.
FIG. 3 shows a construction of the disclosed voltage drop DC-DC converter. In FIG. 3, a voltage converting circuit 9 of the voltage drop DC-DC converter includes a capacitor C1, a capacitor C2, switches SWa, SWb and SWc and a switch control circuit 8 for switching a connection of the capacitors C1 and C2 between a series connection and a parallel connection.
The voltage converting circuit 9 operates to charge the capacitors C1 and C2 having substantially the same capacitance with power of an input voltage Vin when the capacitors are connected in series by on-off switching of the switches SWa and SWb under control of the switch control circuit 8. After the capacitors C1 and C2 are charged, the series connection of the capacitors C1 and C2 is changed to a parallel connection by controlling the switches SWa, SWb and SWc to obtain a voltage across the capacitor C1 as well as the capacitor C2, which is substantially a half of the charged voltage Vin across the series circuit of the capacitors C1 and C2. The thus obtained voltage Vin/2 is transferred to a power capacitor C3 to charge the latter and a power of the charge capacitor C3, that is, charges of the capacitor C3, is outputted externally from an output terminal 9A of the voltage converting circuit 9. Incidentally, a reference numeral 9B in FIG. 3 depicts an input terminal to which the input voltage Vin is applied.
The output voltage at the output terminal 9A of the voltage converting circuit 9 is dropped by a MOSFET Q, which is applied to an output terminal 11A of the DC-DC converter as an output voltage Vout. A gate of the MOSFET Q is connected to an output terminal of a differential amplifier 7 and the output voltage Vout is controlled by an output voltage of the MOSFET Q. The differential amplifier 7 has a plus input terminal supplied with a reference voltage Vref from a reference voltage generator circuit 6 and a minus input terminal supplied with the output voltage from the output terminal 11A of the DC-DC converter and performs a control for making the output voltage Vout at the output terminal 11A becomes equal to the reference voltage Vref.
Although the output voltage Vout at the output terminal 11A of the DC-DC converter is a detection voltage in this case, the voltage converting circuit 9 may output a constant voltage obtained by dividing the voltage Vout by a voltage dividing circuit as the detection voltage.
In such voltage drop DC-DC converter of switched capacitor type, however, there is problems of necessity of a power transistor as the voltage dropping transistor circuit, difficulty of obtaining conversion efficiency of 90% or higher and heat generation and increased area of a power source circuit.
According to this voltage drop DC-DC converter of switched capacitor type, it may be possible to regulate the output voltage by directly controlling the switches for changing the connection of the capacitors by the voltage converting circuit 9, without using the voltage dropping transistor. In such case, however, it is necessary in order to stabilize the output voltage Vout of the DC-DC converter to control the switching frequency or switching period-of the switch control circuit 8. Since, in such control, the switching is performed before the charging of the capacitors is completed, there is a problem that noise may be generated on the output voltage Vout.
An object of the present invention is to provide a voltage drop type DC-DC converter, which is high efficiency with reduced noise generation and occupies a small area.
In order to achieve the above object, the voltage drop DC-DC converter according to the present invention is featured by comprising a first and second capacitors, an oscillator circuit for generating a signal having a specific frequency, a switch circuit including a plurality of switches for alternately connecting the first and second capacitors in series and in parallel in response to the output signal of the oscillator circuit, a DC power source for charging the first and second capacitors with a DC voltage when the first and second capacitors are connected in series, an output terminal for deriving a junction voltage at a junction point of the first and second capacitors when the first and second capacitors are connected in series and deriving a terminal voltage of the first and second capacitors when the first and second capacitors are connected in parallel and a comparator for comparing the voltage at said output terminal with a predetermined reference voltage and generating a signal for stopping the oscillation of said oscillator circuit when the voltage at said output terminal is larger than the reference voltage, wherein the first and second capacitors are connected in parallel when the oscillation of the oscillator circuit is stopped.
In the present invention, it is possible to effectively drop a power source voltage by providing the switch circuit including the plurality of switches for alternately connecting the first and second capacitors in series and in parallel in response to the output signal of the oscillator circuit oscillating at the specific frequency and by deriving the junction voltage at the junction point of the first and second capacitors when the first and second capacitors are connected in series and deriving the terminal voltage of the first and second capacitors when the first and second capacitors are connected in parallel.
Further, since the control of the oscillator circuit, when the aimed voltage is increased beyond the reference output voltage, is to merely stop the oscillation, there is no need of providing a voltage drop transistor, generation of noise and heat of the power source circuit is restricted and an area occupied by the voltage drop DC-DC converter can be small.
As a result, the voltage drop DC-DC converter of the present invention can provide effects of high efficiency, low noise, small heat generation and small occupation area.